Spatial aspects of spin polarization of structurally split surface states in thin films with magnetic exchange and spin-orbit interaction

[EN] A theoretical study is presented of the effect of an in-plane magnetic exchange field on the band structure of centrosymmetric films of noble metals and topological insulators. Based on an ab initio relativistic k . p theory, a minimal effective model is developed that describes two coupled copies of a Rashba or Dirac electronic system residing at the opposite surfaces of the film. The coupling leads to a structural gap at F and causes an exotic redistribution of the spin density in the film when the exchange field is introduced. We apply the model to a nineteen-layer Au(111) film and to a five-quintuple-layer Sb2Te3 film. We demonstrate that at each film surface the exchange field induces spectrum distortions similar to those known for Rashba or Dirac surface states with an important difference due to the coupling: at some energies, one branch of the state loses its counterpart with the oppositely directed group velocity. This suggests that a large-angle electron scattering between the film surfaces through the interior of the film is dominant or even the only possible for such energies. The spin-density redistribution accompanying the loss of the counterpart favors this scattering channel.We acknowledge funding from the Department of Education of the Basque Government (Grant No. IT1164-19) and the Spanish Ministry of Science, Innovation, and Universities (Project No. PID2019-105488GB-I00)

Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: surfaces of bismuth tellurohalides

Spintronics is aimed at active controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction (SOI) in a two-dimensional (2D) electron system immersed in an inversion-asymmetric environment. The SOI induced spin-splitting of the 2D-electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in 2D electron systems which reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Gamma-point energy lying deep inside the bulk band gap. The giant spin-splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations.Comment: 12 pages, 5 figure

New generation of two-dimensional spintronic systems realized by coupling of Rashba and Dirac fermions

Intriguing phenomena and novel physics predicted for two-dimensional (2D) systems formed by electrons in Dirac or Rashba states motivate an active search for new materials or combinations of the already revealed ones. Being very promising ingredients in themselves, interplaying Dirac and Rashba systems can provide a base for next generation of spintronics devices, to a considerabl

Direct observation of many-body charge density oscillations in a two-dimensional electron gas

Quantum interference is a striking manifestation of one of the basic concepts of quantum mechanics: the particle-wave duality. A spectacular visualization of this effect is the standing wave pattern produced by elastic scattering of surface electrons around defects, which corresponds to a modulation of the electronic local density of states and can be imaged using a scanning tunnelling microscope. To date, quantum-interference measurements were mainly interpreted in terms of interfering electrons or holes of the underlying band-structure description. Here, by imaging energy-dependent standing-wave patterns at noble metal surfaces, we reveal, in addition to the conventional surface-state band, the existence of an ‘anomalous’ energy band with a well-defined dispersion. Its origin is explained by the presence of a satellite in the structure of the many-body spectral function, which is related to the acoustic surface plasmon. Visualizing the corresponding charge oscillations provides thus direct access to many-body interactions at the atomic scale

Interplay of surface and Dirac plasmons in topological insulators: the case of Bi2Se3

We have investigated plasmonic excitations at the surface of Bi2Se3(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q∥, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q∥. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q∥∼0.04  Å−1), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi2Se3. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators

Cylindrical antenna array development and measurements for DOA-estimation applications

The article discusses the functioning of the radio direction-finding methods in the system of cylindrical antenna arrays formed of the designed radiators, directional factor of which is not equal to 1. The accuracy of determining the angular azimuth coordinates with the direction-finding method with super-resolution MUSIC is estimated in azimuth plane